1. Field of the Invention
The present invention relates to an automatic focusing system for an adjustable focus lens camera in general, and to such a system having reversible drive means, in particular.
2. Description of the Prior Art
Control systems for automatically positioning the movable element of an adjustable focus lens to a desired focus position in order to properly focus an image of a remote object at the image plane of a photographic camera in response to a signal representative of the distance to said remote object, have been disclosed in the prior art. Control systems that divide the entire operational range of an adjustable focus lens into a plurality of discrete focus zones by generating a plurality of discrete signals (one discrete signal for each focus zone) in order to so focus an adjustable focus lens, have also been disclosed in the prior art.
Prior art control systems having reversible drive means are capable of automatically moving the movable element of an adjustable focus lens in either of two directions to a particular focus zone (as described above) and stopping lens element movement as soon as said lens element reaches said focus zone. If the movable lens element is stopped as soon as it reaches an appropriate focus zone, as determined by a focus zone signal corresponding to said focus zone, said lens element can be positioned to at least two different focus positions for the same focus zone signal, the particular position being dependent upon the end of the focus zone that is entered by said movable lens element. This can result in a lens element positioning differential or focusing error as large as the width of a discrete focus zone.
One fairly common technique for reducing the aforementioned focusing error is to reduce the width of each focusing zone by increasing the total number of focusing zones that collectively represent the entire adjustable focus lens operational range. While this technique does reduce focusing error as discussed above, it does so by increasing the complexity of the automatic focus control system that positions the adjustable focus lens to the appropriate focus zone. For example, increasing the number of discrete focus zones would normally increase the total number of binary coded bits in a digital control system that would be needed to define the additional focus zones for proper automatic focus control system operation. Another technique that might be utilized to reduce focusing error is a ratchet and pawl arrangement where a pawl engages and arrests the movement of a ratchet that is mounted on and rotates with the movable element of an adjustable focus lens. While this type of arrangement would reduce focusing errors of the type described above, such an arrangement often breaks down and is relatively complex and expensive.
In my U.S. Pat. No. 4,178,087, electrodynamic braking is utilized to reduce said aforementioned focusing error by arresting lens movement within a selected portion of the desired focusing zone. While this arrangement is effective at relatively low focusing speeds, the movable lens element tends to pass through or overshoot the desired focusing zone at relatively high focusing speeds which can result in damped oscillatory movement of said movable lens element within said desired focusing zone.
In my U.S. patent application Ser. No. 277 filed on an even date herewith, the above-mentioned high speed focusing zone overshooting problem is resolved by sensing the arrival of the lens at a focus zone immediately adjacent the desired focus zone and disabling the lens drive means for a limited period of time. If the adjustable focus lens is not focused to the desired focus zone during said limited period of time, said lens drive means is enabled until said lens is focused to said desired focus zone. While this arrangement increases the focusing speed of the adjustable focus lens control system, said arrangement does not have the capability of being able to repeatably position the adjustable focus lens to approximately the same relative position within all of the control system focus zones which makes the lens focusing error greater in some focus zones than it does in other focus zones.